The Finger Lakes—a group of long, roughly parallel lakes in upstate New York—got their name for obvious reasons. On a map, the narrow lakes look similar to outstretched digits. Seneca and Cayuga—the two largest Finger Lakes—are among the deepest lakes in North America. Lake Cayuga descends about 435 feet (133 meters) at its deepest point—putting it about 53 feet (16 meters) below sea level.

On May 6, 2013, the Moderate-resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired this true-color image of the Finger Lakes. Water appears dark blue. Farms and orchards, mainly around the northern part of the lakes, are beige. Forested areas south of the lake are brown because leaves had not fully emerged in early May. However, grass and other low-lying vegetation in meadows had turned some parts of the landscape green.

Millions of years ago, the lakes were northward-flowing streams that ran through a series of narrow valleys shaped like a V. Beginning about two million years ago, during a period known as the Pleistocene glaciation, sheets of ice crept south and buried those valleys under ice.

As the glaciers pushed south, they gouged the bottoms and sides of the narrow valleys, deepening and widening them into vertical U shapes. This process, known as glacial scouring, likely happened multiple times as orbital variations in global climate—called Milankovitch cycles—produced cycles of advancing and retreating ice.

The most recent glacial advance occurred about 21,000 years ago, when ice covered much of New York state and New England. The glaciers advanced and retreated, melting for the last time about 10,000 years ago. In the process, they left debris piles called recessional moraines in the valleys. In many cases, these moraines functioned like dams, blocking streams and causing the valleys to fill with water and become lakes.